Several references describing methods for controlling the size of flouride nanoparticles follow. Bender et al., Chem. Mater. 2000, 12, 1969-1076, discloses a process for preparing Nd-doped BaF2 nanoparticles by reverse microemulsion technology. Bender expressly states that aqueous salt solutions such as 0.06 M Ba+2, produce particles smaller than 100 nm while concentrations of about 0.3 M Ba+2 resulting in particles larger than 100 nm. Luminescing particles are disclosed. Bender discloses a decrease in lattice parameter for BaF2 nanoparticles doped with Nd.
Wang et al., Solid State Communications 133 (2005), 775-779, discloses a process for preparing 15-20 nm Eu-doped CaF2 particles in ethanol. Wang expressly teaches away from employing an aqueous reaction medium.
Wu et al., Mat. Res. Soc. Symp. Proc. 286, 27-32 (1993) disclose that CaF2 particles produced by a vapor phase condensation process are characterized by an average particle size of 16 nm while Ca0.75La0.25F2.25 particles prepared by the same process were characterized by average diameter of 11 nm.
Stouwdam et al., Nano Lett. 2(7) (2002), 733-737, discloses synthesis of rare-earth doped LaF3 in ethanol/water solution incorporating a surfactant to control particle size. The resulting produced incorporates the surfactant. 5-10 nm particles are prepared.
Haubold et al, U.S. Patent Publication 2003/0032192, discloses a broad range of doped fluoride compositions prepared employing organic solutions at temperatures in the range of 200-250° C. 30 nm particles are disclosed. The organic solvent employed degrades and acts as a particle-size controlling surfactant.
Knowles-van Cappellen et al., Geochim. Cosmochim. Acta 61(9) 1871-1877 (1997), discloses preparation of 214±21 nm particles by combining in aqueous solution equal volumes of 0.1 M Ca(NO3)2 and 0.2 M of NaF. Knowles-van Cappellen is silent regarding doped particles.
The references teach methods for preparation of muti-valent fluorides, doped and undoped, with particle sizes in the range of about 2 to 500 nm. The teachings are confined to non-aqueous reaction media, or at least water/alcohol. The methods teach various means for controlling the particle size produced. For example, Bender teaches that higher concentrations of reactants lead to larger particles. Others show that the presence of a rare-earth dopant decreases particle size. Others, Stouwdam, op.cit., and Haubold, op. cit., employ surfactants to control particle size.
The present invention represents a breakthrough in the technology of preparing rare-earth doped multi-valent metal fluoride nanoparticles. It is conducted by combining solutions of aqueously soluble salts, preferably at room temperature, and separation of the product by ordinary means including flocculation, filtration, and centrifugation. The rare earth doped particles prepared according to the present invention exhibit luminescence giving them broad utility in many fields while the small particle size reduces light scattering and haze. Applications include use in forming solid state lasers, preparation of optical amplifiers, detectors, coatings, paints and other surface treatment applications.